U.S. patent number 4,140,103 [Application Number 05/813,615] was granted by the patent office on 1979-02-20 for solar energy collectors.
This patent grant is currently assigned to The Broken Hill Proprietary Company Limited. Invention is credited to John M. Leigh.
United States Patent |
4,140,103 |
Leigh |
February 20, 1979 |
Solar energy collectors
Abstract
A solar energy collector comprising a closed envelope formed
from two sheets of metallic material bonded together around their
peripheral edges and spaced apart to define a space adapted to be
filled with working fluid and opening into a vapor space formed
from an outwardly directed indentation in one of the sheets and
into which space working fluid can vaporize when heated by exposing
one surface of the envelope to solar radiation. At least one
condensing tube is provided in direct thermal contact with the
vapor space and through which liquid to be heated flows, while
means are provided for returning condensed vapor from the vapor
space to the working fluid space. In one form of the invention a
lower of the two sheets has a plurality of parallel extending
corrugations formed therein the inner extremities of which abut the
surface of the upper sheet to space the sheets apart, which
corrugations are alternatively of large and small width, the larger
corrugations primarily forming a plurality of working fluid spaces
and the smaller of the corrugations primarily forming a plurality
of condensate return channels, with one condensing tube passing
into and through the vapor space. The closed envelope in one form
of the invention is supported in a container and is surrounded
beneath and at the sides and ends thereof by a layer of insulating
material with the upper open face of the container being closed by
a sheet of toughened glass.
Inventors: |
Leigh; John M. (Kew,
AU) |
Assignee: |
The Broken Hill Proprietary Company
Limited (Melbourne, AU)
|
Family
ID: |
3766698 |
Appl.
No.: |
05/813,615 |
Filed: |
July 6, 1977 |
Foreign Application Priority Data
Current U.S.
Class: |
126/666; 126/675;
165/104.21 |
Current CPC
Class: |
F24S
10/00 (20180501); F24S 10/95 (20180501); F24S
10/90 (20180501); Y02E 10/44 (20130101) |
Current International
Class: |
F24J
2/30 (20060101); F24J 2/04 (20060101); F24J
003/02 () |
Field of
Search: |
;126/270,271
;165/105 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dority, Jr.; Carroll B.
Attorney, Agent or Firm: Cushman, Darby and Cushman
Claims
I claim:
1. A solar energy collector comprising a close envelope formed from
two sheets of metallic material sealed together around their
peripheral edges and spaced apart to define a space adapted to be
filled with a working fluid, said opening into a vapour space into
which said working fluid can vapourise when heated by exposing one
surface of the envelope, defining said working fluid space, to
solar radiation, at least one condensing tube in heat exchange
relationship with said vapour space and through which liquid to be
heated flows, and means for returning condensed vapour from said
vapour space to said working fluid space, wherein at least one of
said two sheets has a plurality of parallel extending indentations
formed therein, the inner extremeties of the walls defining said
indentations abutting the surface of the the other sheet to space
said sheets apart, wherein said indentations are alternatively of
large and small cross-sectional area, said larger indentations
forming a plurality of spaces primarily for working fluid and the
smaller of said indentations forming a plurality of channels
primarily for returning said condensate, and wherein at least one
of said sheets has an outwardly directed indentation formed therein
to define said vapour space, and at least one of said sheets
including an outwardly directed indentation at the opposite end
thereof to provide a channel for fluid communication between said
condensate return channels and said working fluid spaces.
2. A solar energy collector as claimed in claim 1, wherein the, or
each, condensing tube is bonded to the outer surface of the wall of
the part of said envelope defining said vapour space.
3. A solar energy collector as claimed in claim 2, wherein the, or
each, condensing tube is located within an outwardly directed
groove formed in the part of said envelope defining said vapour
space.
4. A solar energy collector as claimed in claim 1, wherein the, or
each, condensing tube passes into and through said vapour
space.
5. A solar energy collector as claimed in claim 1, wherein said
parallel extending indentations are formed by corrugations of
alternatively large and small width.
6. A solar energy collector as claimed in claim 5, wherein the
corrugations are formed in the lower of said two sheets and the
outwardly directed indentation to define said vapour space and the
outwardly directed indentation to provide said channel for fluid
communication between said condensate return channels and said
working fluid spaces are formed in the upper of said two
sheets.
7. A solar energy collector as claimed in claim 1, wherein said
collector further comprises a container open at one face thereof
and in which said closed envelope is positioned, said envelope
being surrounded beneath, and at the sides and ends thereof, by a
layer of insulating material, and wherein the open face of said
container is closed by at least one sheet of solar radiation
transparent material.
8. A solar energy collector as claimed in claim 7, wherein the
transparent material is toughened glass.
9. A solar energy collector as claimed in claim 1, wherein said
working fluid is water.
Description
This invention relates to improvements in solar energy
collectors.
The conventional "flat plate" solar collector basically comprises a
series of vertical copper tubes, spaced at intervals with about 150
mm, connected to top and bottom horizontal tubes which provide flow
passages for the water being heated. The vertical tubes are usually
covered with a thin copper sheet which is surfaced with either
black paint or a more efficient selective coating so that the
copper sheet acts as the principal solar radiation absorber. The
collected energy is conducted from the copper sheet to the water in
the copper tubes and the hot water in the collector is usually
transferred to the storage tank by thermo-siphon, the hot water
rising to the storage tank and forcing the colder water down to the
collector to be heated.
Because of the comparatively large mass of water in the collector
tubes, heat transfer between the collector and storage is
relatively slow; consequently quite substantial amounts of
collected energy can be lost during periods of intermittent
sunshine. Another major disadvantage of conventional collectors is
that because the system is "open", corrosion prevention is the
major consideration in material selection. Hence expensive copper
tube and sheet is required with most present collector designs.
Furthermore, conventional collectors are limited by their design to
a maximum water temperature below 100.degree. C. for example, about
80.degree. C.
Generally, the efficiency of a solar system depends on the
differential between the operating and ambient temperatures. It
will be evident from the above that in places with a higher ambient
temperature, the existing commercial solar absorbers have serious
limitations. The object of this invention is to provide a solar
energy collector that overcomes most of the limitations of
conventional collectors and can operate more efficiently.
A solar energy collector comprising a close envelope formed from
two sheets of metallic material sealed together around their
peripheral edges and spaced apart to define a space adapted to be
filled with a working fluid, said space opening into a vapour space
into which said working fluid can vapourise when heated by exposing
one surface of the envelope, defining said working fluid space, to
solar radiation, at least one condensing tube in heat exchange
relationship with said vapour space and through which liquid to be
heated flows, and means for returning condensed vapour from said
vapour space to said working fluid space, wherein at least one of
said two sheets has a plurality of parallel extending indentations
formed therein, the inner extremeties of the walls defining said
indentations abutting the surface of the other sheet to space said
sheets apart, wherein said indentations are alternatively of large
and small cross-sectional area, said larger indentations forming a
plurality of spaces primarily for working fluid and the smaller of
said indentations forming a plurality of channels primarily for
returning said condensate, and wherein at least one of said sheets
has an outwardly directed indentation formed therein to define said
vapour space, and at least one of said sheets including an
outwardly directed indentation at the opposite end thereof to
provide a channel for fluid communication between said condensate
return channels and said working fluid spaces.
In one form of the invention the condensing tube extends into, and
passes through said vapour space. In another form, several
condensing tubes are engaged in grooves formed in the wall of the
envelope at said vapour space and are intimately bonded to said
envelope so that thermal conduction to the tubes is maximum.
The envelope is preferably formed by two sheets of metals such as
stainless steel suitably sealed around their peripheries and spaced
to define said working fluid space. The vapour space is preferably
defined by an indentation in one or both sheets.
Three preferred forms of the invention will now be described with
reference to the accompanying drawings in which:
FIG. 1 is a plan view of the upper component of one preferred form
of solar energy collector envelope incorporating the present
invention,
FIG. 2 is a cross-sectional view taken along line 2--2 of FIG.
1,
FIG. 3 is a plan view of the lower component of the collector
envelope which is adapted to be joined to the component of FIG. 1
to form the complete envelope of the collector,
FIG. 4 is a cross-sectional view taken along line 4--4 of FIG.
3,
FIG. 5 is a cross-sectional view of part of a complete collector
envelope taken along line 5--5 of FIG. 3,
FIG. 6 is a plan view of the upper component of a second preferred
form of solar energy collector envelope incorporating the present
invention,
FIG. 7 is a plan view of the lower component of the collector
envelope which is adapted to be joined to the component of FIG. 6
to form the complete envelope of the collector,
FIG. 8 is a cross-sectional view of portion of the complete
collector of this embodiment taken along line 8--8 of FIG. 6,
FIG. 9 is a plan view of the upper component of a third preferred
form of solar collector envelope incorporating the present
invention,
FIG. 10 is a cross-sectional view taken along line 10--10 of FIG.
9,
FIG. 11 is a plan view of the lower component of the collector
envelope which is adapted to be joined to the component of FIG. 9
to form the complete envelope of the collector,
FIG. 12 is a cross-sectional view taken along line 12--12 of FIG.
11,
FIG. 13 is a cross-sectional view taken along line 13--13 of FIG.
11,
FIG. 14 is a perspective view of one form of complete solar energy
collector incorporating a solar energy collecting envelope of the
type illustrated with reference to FIGS. 9 to 13,
FIG. 15 is a cross-sectional view taken along line 15--15 of FIG.
14,
FIG. 16 is a cross-sectional view of part of the collector taken
along line 16--16 of FIG. 14,
FIG. 17 is a graph showing comparative efficiencies for one
preferred form of collector according to the present invention and
that for a conventional collector, and
FIG. 18 is a graph showing variations in values of relevant
information derived from tests carried out on a collector according
to the present invention.
With reference to FIGS. 1 to 5 of the drawings, the collector of
this first embodiment comprises two thin (0.5 to 1 mm) sheets of
steel 1 and 2, the top sheet 1 being treated on its outer surface
to provide a selective solar energy absorber surface S. For
example, the sheet 1 may be treated with CSIRO chrome black or any
other suitable treatment.
The bottom sheet 2 is formed with a plurality of upstanding ribs 3
by means of which a suitable working fluid space (about 0.5 mm
deep) is formed and maintained between the two sheets 1 and 2 which
are seamed together around their peripheries in the same manner as
is used in can end seaming to provide an hermetic seal 9. The
bottom sheet 2 is also formed with a plurality of interconnected
outwardly directed elongated indentations 4 which define with the
top sheet 1 a plurality of condensate return channels. The channels
include side channels 5 extending down the sides and opening into a
bottom channel 6 which is open to the space between the sheets 1
and 2. Across channel 7 extends across the plate 2 between the side
channels 5 and a plurality of short feeder channels 8 extend up
from the cross channel 7.
The channels 7 and 8 are open to a vapour space 10 formed by an
outwardly directed depression in the top sheet 1. The depression is
formed with several outwardly directed channels 12 which are
adapted to substantially surround condensing tubes 11 of copper or
like corrosion resistant material of good heat conductivity. To
ensure good heat conduction between the tubes 11 and the portion of
the sheet defining the vapour space 10, the tubes 11 are soldered
or otherwise intimately bonded to the top sheet 1.
Prior to the final sealing of the collector the space is evacuated
to about 25 mm of water through a hole (not shown) and the space is
filled with water or any suitable liquid (e.g. to the level of the
vapour space 10) and then hermetically sealed. While evacuation is
not essentials for the collector to work, the efficiency of
operation is improved substantially thereby. It may be necessary to
weld the sheets 1 and 2 together about their peripheries, or some
suitable sealing compound positioned between the seam between the
two sheets to ensure a hermetic seal.
In use, the condensing tubes 11 are connected to a tank of water
and the absorber surface S is exposed to solar radiation and the
collector supported at an inclined position. Whenever the surface S
is heated above the temperature of water in the tank, the water in
the working fluid space boils generating vapour in the vapour space
10 which condenses on the portions of the sheet 1 in contact with
the tubes 11 to heat the water therein. The tubes 11 are inclined
at about 3.degree. to 4.degree. to the horizontal to ensure
positive movement by thermosiphon of the water in the tubes. The
vapour condenses on the inside walls of the vapour space 10 and
runs down the channels 8 into the channel 7 and thence into
channels 5 to channel 6 where it is available for vapourization.
This process continues until the liquid in the tank attains the
temperature of the absorber surface S. In the second embodiment
shown in FIGS. 6, 7 and 8, which embodiment represents a
modification of the embodiment of FIGS. 1 to 5 and where applicable
the same numerals are used, the ribs 3 are replaced by dimples 20
formed in the bottom sheet 2. Also, the indentation 21 defining the
vapour space is flat bottomed and a condensing tube 22 extends
through the bottom plate 2 and into the vapour space where it is in
direct contact with the vapour and vapour condenses directly on the
wall of the tube 22. For this reason this second embodiment is more
preferred than the first embodiment. Otherwise the arrangement and
operation is identical to the first embodiment.
In the third embodiment, with reference to FIGS. 9 to 13, the
collector also comprises two thin sheets of stainless steel 30 and
31, the top sheet 30 being treated on its outer surface to provide
a selective solar energy absorber surface S as with the preceding
embodiments. The bottom sheet 31 has a plurality of alternatively
large and small width parallel corrugations 32 and 36 respectively
formed therein, extending side by side across substantially the
full width of the sheet 31, with each corrugation extending from a
position adjacent a lower edge of the collector, when inclined, to
a position adjacent the upper edge of the collector. The bottom
sheet has a peripheral flange 33 formed thereon, a portion 33a of
which at one end of the sheet is greater in height than the
remainder of the peripheral flange. The upper sheet 30 also has a
peripheral flange 34 formed thereon, and when the top and bottom
sheets 30 and 31 are united they are joined by welding or seaming
their peripheral edges 33 and 34 to provide a hermetic seal. The
two sheets may be welded together at intervals sufficient to
produce a structure capable of withstanding a positive internal
pressure in the envelope of 4 to 5 atmospheres. When the two sheets
30 and 31 are joined the inner ridges of the corrugations 32 and 36
in the lower sheet abut the inner surface of the upper sheet 30 to
form channels 35 approximately 0.6 mm deep within the larger
corrugations 32 primarily forming working fluid spaces and smaller
channels 37 within the smaller corrugations 36 primarily acting as
condensate return channels.
The channels 35 and 37 are open to a vapour space 38 formed by an
outwardly directed depression in the top sheet 30, and are open at
their opposite ends to a bottom channel 39 formed by a slightly
outwardly directed depression in the top sheet 30. It is believed
that vapour will tend to form in the larger channels 35 insofar as
they offer the path of least resistance, whilst any fluid
accumulating in the vapour space 38, either as condensate or fluid
carried by the vapour up the channels 35, will return back down the
smaller condensate return channels 37. As shown, the vapour space
38 formed by the outwardly directed depression in the top sheet 30
has sloping end walls 40 through which a condensing tube 41 passes
to extend through the vapour space 38. The condensing tube is
affixed to the sloping end walls 40 by silver soldering.
As with the previous embodiments, prior to final sealing of the
collector envelope, the space is evacuated to about 25 mm of water
through a hole (not shown) and the space is filled with water or
suitable fluid (e.g. to the level of the vapour space 38) and then
hermetically sealed. In use, the condensing tube 41 is connected to
a tank of water and the absorber surface S is exposed to solar
radiation and the collector supported at an inclined position. As
with the previous embodiments, when the surface S is heated above
the temperature of water in the tank, the water or fluid in the
working fluid channels 35 boils generating vapour in the vapour
space 38 which condenses on the outer surface of the condensing
tube 41 within the space to heat the water within the tube 41. The
tube 41 may be inclined at about 3.degree. to 4.degree. to the
horizontal as with the previous embodiments to ensure positive
movement by thermo-siphon of the water in the tube. The condensed
vapour runs down the condensate return channels 37 into the bottom
channel 39 and then into the channels 35 where it is available for
vapourization. This process continues until the liquid in the
storage tank attains the temperature of the absorber surface S.
Referring to FIGS. 14 to 16 of the drawings there is shown a
complete solar energy collector, generally indicated as 42, and
incorporating the solar energy collecting envelope of the
embodiment of FIGS. 9 to 13 which is identified by the same
reference numerals.
The complete collector comprises a shallow box shaped container
open at the top and formed from sheet zincalume which is cut and
folded to form a base wall 43, longitudinal side walls 44, end
walls 45, and inturned upper peripheral edges 46. The top of the
container is closed by a lid member 47 comprising a peripheral
frame 48 of L-shaped section adapted to overlie the edge of the
container as shown, and, as shown, a sheet of toughened glass 49 is
adapted to be received between the inturned edges 46 of the
container and the peripheral frame 48, whilst supporting strips of
resilient material 50 are interposed between the glass sheet 49 and
the container.
The solar energy collecting envelope is positioned within the
container as shown in FIGS. 15 and 16, and is surrounded beneath,
and at its sides and ends, by a layer of insulating material 51,
such as, fibreglass, whilst in order to retain the collector
envelope in position, particularly when the collector is tilted or
inclined during use, the upper edge of the higher portion of the
peripheral flange 33a bears against the downwardly inturned section
of the adjacent upper peripheral edge 46 of the container, whilst
one longitudinal edge of a connecting strip 52, with a rolled edge
53, is attached to one side of the portion of the collector
envelope which defines the vapour space 38, with the rolled edge 53
being embedded within the adjacent layer of insulating material 51
as shown.
A test has been carried out on a solar energy collector of the type
described and illustrated above with reference to FIGS. 9 to 16 of
the drawings, and the results of the test compared with the
characteristics of a typical conventional collector in order to
provide an indication of what degree of improvement a solar energy
collector of the present invention would offer over a conventional
collector.
For the purposes of the test a simplified form of solar energy
collector was fabricated which differed from the collector
described above insofar as the collector envelope was manufactured
from sheet aluminium, whilst a 12 mm diameter copper condensing
tube was utilised. The upper surface of the collector envelope was
coated with a matt black paint, and the envelope was supported in a
container constructed from timber with a 25 mm thickness of
polystyrene insulation backing the envelope and the collector was
finally completed with a 3 mm thick toughened glass cover.
Furthermore, a thin strip of metal twisted to form a spiral was
inserted inside the condensing tube to induce greater turbulence in
the fluid (in this case water) passing through the tube. The
working fluid within the collector envelope was water.
The output end of the condensing tube of the collector was, via a
pump, attached to, and in communication with the interior of, a
storage tank filled with water at a position adjacent the top
thereof, whilst the opposite end of the condensing tube was
attached to, and in communication with the interior of, the storage
tank at a position adjacent to the bottom thereof.
During the test, temperature readings were taken, and recorded, for
the top and bottom of the water storage tank, whilst readings were
simultaneously taken and recorded for ambient temperature and the
temperature of the collector envelope. The temperatures of the
water in the storage tank, and ambient temperature, were measured
with glass mercury thermometers, whilst the collector envelope
temperatures was measured by thermocouples. The amount of incident
solar radiation (insolation) was also simultaneously measured and
recorded using a Middleton Pyranometer, which was set and inclined
at the same angle of inclination as the test collector and the
collector and pyranometer were located on a stand generally facing
the northern sky, which at the time of the year in which the test
was carried out in Australia, is where the sun is situated.
The test was carried out between 1145 and 1300 hours on the day
selected for the test, and comprised 16 separate readings at
intervals of about 6 minutes.
From the readings taken the efficiency calculations for the
collector were made in accordance with the following formula:
##EQU1## where: .eta. = efficiency
M.sub.1 = mass of water in storage tank
M.sub.2 = mass of storage tank
M.sub.3 = mass of fittings
Cp.sub.1 = specific heater of water
Cp.sub.2 = specific heat of storage tank
Cp.sub.3 = specific heat of fittings
A.sub.c = area of glass aperture for the collector
G = insolation
.DELTA..sub.t = increment of temperature
It is important to note that during the test heat losses from the
pump, pipes and tanks have not been allowed for in the calculations
made. The readings and calculations made are set out in table 1, in
which T.sub..omega. is the average temperature in the water storage
tank calculated from the temperature measurements made at the top
and bottom of the tank, and T.sub.e is the approximate sky
temperature which is equivalent to the ambient temperature minus
3.degree. C.
TABLE I
__________________________________________________________________________
Average Efficiency Ambient Tank Insolation Time Averaged Temp Tank
Temp .degree. C Temp G Increment T.sub..omega. - Te Efficiency Over
.degree. C Top Bottom T.sub..omega. .degree. C W/M.sup.2 Min G
.eta. Two Periods
__________________________________________________________________________
15 26.0 25.5 25.75 15 28.0 27.5 27.75 868 6 0.018 0.71 16 29.5 29.0
29.25 904 6 0.018 0.51 0.61 15.5 30.5 30.0 30.25 901 5 0.02 0.41
15.5 33.0 32.5 32.75 922 6 0.022 0.83 0.62 15.5 34.5 33.5 34.0 912
5 0.023 0.50 15.5 36.0 35.2 35.6 909 6 0.025 0.54 0.52 16.0 37.5
36.6 37.15 933 5 0.026 0.61 15.5 39.0 38.5 38.75 897 5 0.029 0.65
0.63 15.5 40.0 39.5 39.75 916 6 0.029 0.32 15.5 41.5 41.0 41.25 890
5 0.031 0.62 0.47 16.0 43.0 42.0 42.5 885 6 0.033 0.43 16.5 44.2
43.5 43.85 857 5 0.036 0.58 0.51 16.5 45.2 44.5 44.85 890 5 0.035
0.41 16.0 46.5 45.8 46.15 890 6 0.037 0.45 0.43 16.0 47.0 46.7
46.85 618 5 0.055 0.42 16.0 47.0 47.0 47.0 123 5 0.27 0.45 0.43
__________________________________________________________________________
From Table I it will be observed that the efficiencies were also
averaged over two intervals to reduce scattering of the efficiency
values when plotting the values on the graph constituting FIG. 17
of the drawings.
At this stage, as far as can be determined, there is no standard
test procedure for testing solar energy collectors, and as such
some difficulty was encountered in locating test results for a
conventional solar energy collector in literature available, to
enable a meaningful comparison to be made between the results of
the testing of the collector of the present invention and a
conventional collector.
However, reference has been made to a technical report No. TR9
published by the Commonwealth Scientific and Industrial Research
Organization (CSIRO) in Australia.
The efficiency curve for the conventional collector shown in the
CSIRO technical report has been represented as a solid line in
FIGS. 17 of the accompanying drawings, which curve from the CSIRO
report was produced from test data from a collector believed to
have a selective coating designed to reduce heat losses and improve
collector efficiency. According to the CSIRO report their test was
conducted during a clear sunny day between 1100 and 1300 hours,
whilst the collector was reorientated at ten minute intervals so
that the surface of the collector always remained normal to
incident radiation to ensure minimal reflective losses from the
glass cover for the collector. The chain-dot line in the graph of
FIG. 17 of the drawings represents an estimate of the actual
efficiency when the thermal mass of the collector tested by CSIRO
was taken into account, and it should also be noted that the method
used for the test of the collector of the present invention did not
take into consideration the energy used to heat the collector
itself.
From the test data for the collector of the present invention when
compared with that of the conventional collector of the type the
subject of the CSIRO report, and with reference to the graph of
FIG. 17, it will be observed that the efficiency of the collector
of the present invention compares favourably with that of the
conventional collector having a selective coating, and it would be
reasonable to expect significant improvements in efficiency for the
collector of the present invention once a selective coating is
applied to the surface of the collector envelope.
Referring to the graph constituting FIG. 18 of the drawings, a
comparison between insolation, water storage tank temperature, and
ambient and collector envelope temperatures for the collector of
the present invention, has been made, from which it will be
observed that the temperature differential between the collector
envelope temperature and the average temperature of the water in
the storage tank remains at about 8.degree. C. for a period of
relatively constant insolation.
As a rule, when the characteristics of a collector according to the
present invention are compared with those of a conventional solar
energy collector, it has been observed that the collector envelope
may be manufactured from any suitable metal provided care is taken
with the choice of the working fluid to be used in the collector,
whilst with conventional collectors copper is normally only
suitable. Furthermore the collector of the present invention is
suitable for mass production, whilst the production of conventional
collectors is highly labour intensive, whilst furthermore the
collector of the present invention has a lower thermal mass than
that of a conventional collector. It has also been observed that a
collector according to the present invention is suitable for all
climates provided that once again a suitable working fluid is used,
whilst with conventional collectors most are proved to be
unsuitable for climates where ambient temperatures fall below
0.degree. C., because freezing of the water in the tubes causes
fractures of joints between the various components. As a rule
pressure drop through the collector envelope with the collector of
the present invention is lower than that for conventional
collectors, and the collector of the present invention is suitable
for use with mains pressure water systems, which is not the case
with conventional collectors. It has also been observed that
cleaning of the condensing tube incorporated in the collector of
the present invention is simple, as compared with the extreme
difficulty normally encountered in cleaning the tubes within a
conventional collector once they become fouled. In a situation
where a plurality of collectors are combined to form a total
collector system, when collectors of the present invention are
utilised failure of a single collector within the system usually
has a minimal effect on the operation of the system as a whole,
whilst with conventional systems failure in a single collector
usually effects the complete system.
With the collector of the present invention the collector usually
acts as a thermal diode which permits heat transfer in one
direction only, whilst when there is no vapour surrounding the
condenser tube it is effectively enclosed in a vacuum and thus only
very small heat losses will occur. However, conventional systems
will thermo-siphon unless well designed check valves are installed
within the system.
In summary, with each of the embodiments described and illustrated
above, the following major advantages are believed to be
present:
(a) Because the heat transfer system employed in the invention is
"closed", it is possible to use working fluids other than water. In
this way corrosion problems can almost be eliminated and few
restrictions remain with respect to the selection of materials for
the collector construction.
(b) Liquid storage temperatures approaching the temperature of the
collector envelope (110.degree. C. to 120.degree. C.) should be
possible provided that suitable working and storing fluids are
used. The attainment of higher temperatures is greatly dependent on
the selectivity of the surface for the collector envelope, as
described by Horowitz and Watson-Monroe in "A new selective
surface", in the June 1973 Proc. Sym. Int. S.E. Soc. Anz, and heat
losses from the collector surface.
(c) Because of the low thermal mass of the system and the rapid
heat transfer from collector to storage, more efficient use should
be made of sunlight in days of intermittent cloud cover.
(d) Because of the structural strength and simplicity of the
collector according to the present invention they can be readily
integrated with existing building roofs or form complete roof
systems.
* * * * *